mudbaydude wrote:
Thanks folks. Great forum response, as usual. This Sunseeker does have a full-size spare mounted under. The only TPMS is the single warning light in front. Any idea what PSI triggers this light, and is it reasonably accurate?
I'm checking around today to maybe have a full TPMS installed.
There are a couple of different technologies used for TPMS systems, and I don't know offhand which one you may be using. Some systems have actual pressure sensors for each tire, and some sort of a low-power RF interface to get signals from the sensors to the vehicle. These are pretty accurate, about as accurate as a typical digital tire pressure gauge, although some are only set up to register above or below some threshhold (i.e. the sensors only communicate a "good" or a "bad" signal rather than the actual pressure value). The nicer systems have a display available on the dashboard that gives you the tire pressures in each of the tires, or at least an indication of which one is low, and I've even seen where some have a mode for adding air wherein the vehicle lights/horn will blip when you reach the proper pressure. The drawback with these systems tends to be the sensors, which usually have batteries that eventually go dead and seem occasionally prone to damage otherwise. I believe this is how all (or virtually all) aftermarket TPMS systems work.
The other major system design doesn't sense pressure directly at all, but instead relies on the wheel rotation sensors for the ABS system to compare the relative wheel rotation speeds for each wheel and indirectly detect a low tire based on it being slightly smaller in diameter and thus rotating faster than it should. These systems have some button or something to reset/relearn the proper tire sizes that must be pressed whenever the tires are changed or air pressure is adjusted. I rather doubt a vehicle with dual wheels would rely on this kind of system, since it would presumably have a hard time detecting when one of the dual wheels is low, but perhaps it still works out. The advantages of this system are that it's cheap (the sensors are already there, so it's just software in the vehicle computer), and there isn't any additional bits to break down. There are several disadvantages: they often cannot detect when multiple tires are low simultaneously, and usually can't tell which tire is low, and need to be reset from time to time. They're probably also less accurate in overall terms. If you're a vehicle maker, though, and the government says "thou shalt have TPMS," the low cost is very, very appealing.
